This invention relates generally to turbine blades, and more particularly to rotating blades for a turbine.
In turbines, e.g., steam turbines, a plurality of rotating arrays of foils or blades are arranged circumferentially about a rotor. Reaction of steam or gases against the blades produces rotation of the rotor and associated blade arrays. The forces acting on these rotating blades, including centrifugal forces caused by rotation, tend to throw the blades radially outward and generate large forces on the blade attachment structure. In many turbines, the attachment structure comprises a Christmas tree shape root structure which slides into a mating slot or groove in the rotor. When the blade is properly positioned, lugs extending from opposing sides of the root structure share equally in retaining a blade to the rotor, however, if a blade is aligned off of a radial line of the rotor through the center of the rotor groove, lugs on one side of the root structure may support more force than those on the opposite side leading to potential overstress conditions. Accordingly, it is desirable to assure that turbine blades are aligned with their respective center lines oriented along radial lines of the turbine rotor.
Such arrays of rotating blades are often joined together at their tip by a shroud ring which are normally riveted to the blade via a tenon made integral with the blade. The tenon being an abrupt change in cross-section of the blade is subject to higher stresses due to bending moments imposed by the shroud ring and provides crevices wherein corrosion products are accumulated; provide a steam seal over the top of the blade and also however, the shroud rings greatly reduce blade vibration. One prior art turbine blade which eliminates the problems of tenon stress and corrosion is disclosed in U.S. Pat. No. 4,533,298, issued to Albert J. Partington et al on Aug. 6, 1985, assigned to the assignee of the present invention, and incorporated herein by reference.
Partington et al. teaches a plurality of rotatable blades disposed in a circular array, each blade comprising a root portion, which fastens the blades to the rotor, an airfoil shaped blade portion having a leading edge and a trailing edge, and a shroud portion made integral with the blade portion and disposed on the radially outer end of the blade portion. The shroud portion has a leading planar surface and trailing surface, one of the planar surfaces being disposed generally parallel to an axial radial plane passing through the central portion of the root portion, and the other planar surface if extended forming an angle with the radial axial plane passing through the center of the root portion generally equal in degrees to 360 divided by the number of blades forming the circular array. Such turbine blades are disclosed in U.S. Pat. No. 4,602,412, also issued to Partington et al on July 29, 1986, assigned to the assignee of the present invention, and incorporated herein by reference.
Requirements of strict radial alignment imposed to prevent unequal loading on the sides of a turbine blade root portion, and tight contact between the respective shroud to minimize vibration during full-speed operation and wear of the root portions during turning gear operation have necessitated a change in the approach to assembling rows of turbine blades having integral shrouds.